G. Bordin

Nanocrystallization of ferromagnetic Co-rich amorphous alloys and magnetic softening

Ultra-fine grain structures were obtained in soft Co-based amorphous alloys subjected to suitable heat treatments within a restricted range of temperatures which begins well below the conventional crystallization temperature, , of the alloy. These structures lead to superior soft magnetic properties and consist of magnetic nanocrystallites with an average grain size in the range 10 - 12 nm embedded within a residual ferromagnetic amorphous matrix. The results refer to amorphous ribbons of Vitrovac 6025, which were obtained by the single-roller quenching technique and had the nominal composition . With regard to the coercive field and the initial reversible permeability, the best softening nanocrystallization effects occurred in toroidal samples heat treated for about half an hour at around . For the amorphous alloy, the nominal values of (the Curie temperature) and are 210 and , respectively. The results are interpreted on the basis of the random anisotropy model generalized to two-magnetic-phase systems.

Temperature dependence of magnetic properties of a Co-based alloy in amorphous and nanostructured phase

The magnetic properties of a Co-based amorphous alloy (Metglas 2714A) are compared with those of the corresponding nanostructured alloy in the temperature range 30 K to the Curie temperature. A minimum appears in the permeability versus temperature curve well below the Hopkinson peak for the amorphous alloy. This minimum is ascribed to a form of induced anisotropy due to structural instabilities. In contrast, the alloy containing the nanostructured phase, having excellent soft magnetic properties, shows a maximum below the Hopkinson peak which is interpreted as a combination of the variations of the anisotropy and the saturation magnetization with temperature. The anisotropy field distribution is also evaluated in the low-temperature range for both the amorphous and nanostructured phase of the alloy. The most probable anisotropy field for the nanostructured sample is ten times lower than that of the amorphous sample.

Influence of the structural instabilities on the anisotropy fields in Co-based amorphous alloys

Abstract The influence of structural instabilities, produced by thermal treatments on the magnetic anisotropy and initial permeability, is examined in a set of Co-based amorphous alloys. The analysis is made on square strips cut out from as-received ribbons in order to study the magnetic properties along two in-plane perpendicular directions having the same demagnetizing factor. Generally, in the as-received state the samples show an easy magnetization direction parallel to the original longitudinal direction of the ribbon. Annealing, performed at various temperatures, in the absence of applied magnetic fields and mechanical stresses, enhances the original anisotropy. Some exceptions are observed: in one of these alloys the influence of the annealing is very weak, whereas in another alloy the annealing treatment gives rise to a change of the easy axis from the longitudinal direction to the in-plane transverse direction. The evolution of the anisotropy field distribution with the annealing features is also evaluated. The results are discussed on the basis of irreversible changes of local atomic order.

Peculiar behavior of the magneto-impedance in Fe-based amorphous wires under torsion

Abstract The inductive ( ωL ) and resistive ( R ) components of the impedance of an amorphous wire (Fe 77.5 Si 7.5 B 15 ) subjected to torsional stresses are measured. For a critical value of the AC driving current L abruptly increases due to irreversible magnetization processes in the circumferential configuration. Suitable combinations of the AC driving current amplitude and frequency give rise to a peak in the resistive variations and a minimum in the inductance. The inductance shows a bistable asymmetric hysteresis loop ( L vs. H ). The torsional stress increases the coercivity of the longitudinal hysteresis loop of the wire but favors the reaching of the saturation magnetization at lower fields.

Temperature dependence of magnetic properties and phase transitions in a soft magnetic Co-based nanostructured alloy

We report the temperature dependence of magnetic properties and phase transitions in a soft-magnetic Co-based nanostructured alloy obtained from amorphous Vitrovac 6025. Homogeneous nanostructured samples are achieved by suitable thermal treatments below the conventional crystallization temperature and have ultrafine grain structures with average grain size of about 10 nm and volume fraction 0.6. The grains are embedded in a residual ferromagnetic amorphous matrix and this leads to superior soft-magnetic properties which, in our view, can be justified on the basis of the random anisotropy model. The temperature dependence of initial reversible permeability and coercive field emphasizes a different behaviour in the magnetic properties of nanostructured and amorphous samples. The sharpness in the Hopkinson peaks of the initial permeability is the best tool to show the first magnetic transition in both specimens: the results are compared with those detected by thermogravimetric curves and by magnetization intensity measurements. Differential thermal analysis is used to evaluate the transformation enthalpy of the amorphous-to-microcrystalline and the nano-to-microcrystalline processes, which both occur at about 820 K where another ferromagnetic phase appears.

Hall effect and magnetoresistance in Co-based amorphous and crystallized alloys

Abstract We have analyzed the crystallization effects on the Hall resistivity and magnetoresistance in two different sets of Co-based amorphous glasses having a similar composition and commercially known as Metglas and Vitrovac. The behaviours of the Hall constants R o and R S show that the mobility μ of the conduction electrons of the crystallized phase is improved in comparison with that of the amorphous phase and at the same time the scattering effects are reduced. Moreover, the ferromagnetic anisotropic resistivity (FAR) increases with the Co percent content and decreases with resistivity while this does not occur in crystallized samples. In the latter, the longitudinal and transverse variations of resistivity with applied magnetic field are equivalent, but the values of the FAR are much higher than those of the amorphous state. The behaviour of the nanostructured samples is quite similar to that of the amorphous state.

Magnetoelastic effects in soft nanocrystalline Fe-based alloys

Abstract Nanocyrstalline Fe-based alloys of Finemet are here investigated. These ferromagnets were prepared by annealing amorphous ribbons and have excellent soft magnetic properties. Ribbons prepared by zero-field annealing and by annealing in longitudinal or transverse magnetic fields are considered. A comparison is made between the anisotropic effects induced at room temperature by applying external stresses and those induced by the magnetic fields applied during the production process. The analysis is performed through the characteristics of the hysteresis loops, the distribution function of the anisotropy fields together with measurements of the initial complex permeability and of the magnetoresistivity. The results have been interpreted on the basis of the random anisotropy model.

Magnetoresistance and magnetic properties in amorphous Fe-based wires

Abstract The longitudinal and transverse magnetoresistances in amorphous Fe 77.5 Si 7.5 B 15 wires are studied at different values of the DC-bias currents in order to clarify the mechanism of the magnetization according to a ‘core-shell’ domain model. The role of closure domain structures in the magnetization process of the wires is analysed. Moreover, the effects of the Joule heating on the internal stresses, introduced during the rapid quenching in the sample preparation, are examined.

Influence of the chemical etching on magnetic and electric properties of Co-rich amorphous sheets

Abstract The anisotropy characteristics of amorphous strips cut out from a Metglas 2714A ribbon and uniformly thinned by chemical etching are considered. Magnetic and electric properties are measured in the direction of the original ribbon axis and perpendicular to it, in the strip plane. Thinned samples are analyzed also after annealing above the Curie temperature, where annealing in the absence of an applied magnetic field and external stress gives rise to an unusual change in the easy axis direction. The results show that the defects mostly responsible for the anisotropy are prevailingly concentrated near the wheel-side and the air-side of the ribbon with different distributions on the two surfaces.

Effects of atomic diffusion processes in Co-Cu multilayer granular films

Co-Cu multilayer granular films deposited onto SiO2/Si substrates have been submitted to thermal treatments in scanning mode. Their magnetisation, resistance and magnetoresistance have been studied after applying two different heating rates. The analysis of experimental data points out that only with the lower heating rate the properties of the films are strongly modified. In this case, indeed, XPS measurements show the presence of interdiffusion between atomic species of the substrate and metallic species inside the film. XPS data are presented to show the modifications in composition and chemical environment produced by thermal treatments.